This invention relates to sports and has particular relationship to tennis balls. Tennis balls are made of natural or synthetic rubbers or of combinations of natural and one or more synthetic rubbers. A tennis ball has an inner-rubber core which is formed by mating and sealing two hemispherical sections of the rubber together by heating and vulcanization. After the inner-rubber core is formed, an outer cover is adhered to the core. This cover is formed of at least two pieces of felt-covered material having high abrasion resistance which are mated together. The outer cover is typically composed of wool or synthetics such as NYLON or DACRON fabric, or the like.
There are two principle types of tennis balls in use currently: the pressurized type and the unpressurized type. The pressurized type is produced by increasing the pressure within the core to about 14 pounds per square inch guage (about 2 atmospheres) after the core is formed. The pressure within the core of an unpressurized ball is about 1 atmosphere. To improve its characteristics the unpressurized balls have a greater wall thickness of the core, i.e., a greater wall thickness of the resilient elastomeric material. The making of the pressurized ball requires the pressurizing step which is not required in the unpressurized ball. The unpressurized ball demands more elastomeric material. On the whole the pressurized balls are more lively than the unpressurized balls and are preferred; however, they have the disadvantage that they must be packaged in pressurized cans and that the balls lose pressure when removed from the containers and, therefore, become less lively with time. Also, the balls lose pressure if the pressure cans are kept for a long time.
To improve the liveliness of the balls of both types so that they meet demanding standards, tennis balls are often made of natural rubber which is more costly than synthetic rubber.
This invention deals with the liveliness of the balls. Scientifically, the property of elastomers which determines liveliness is called dynamic resiliency. Dynamic resiliency is defined as the ratio of the vertical height of the first rebound of a falling object to the vertical height of the first fall. Damping or hysteresis, which is used to measure deadness, the opposite of liveliness, is proportional to one minus dynamic resilience. It is desirable that the dynamic resilience of tennis balls should be between 0.53 and 0.58.
It is an object of this invention to improve the dynamic resilience of a tennis ball and particularly of an unpressurized tennis ball. It is also an object of this invention to improve the abrasion resistance of the covering of a tennis ball.